Resolving electrolayers from VES : a contribution from modeling the electrical response of a tightly constrained alluvial stratigraphy

The reliability of the hydrostratigraphic interpretation of electrostratigraphy derived from ground based, Direct Current resistivity methods is analyzed through the forward modeling of synthetically derived electrostratigraphic layering in a tightly constrained alluvial framework. To this purpose, a high-resolution stratigraphic model of the horizontally-stratified, alluvial aquifers hosted by the Quaternary regressive cycle of the Po plain in Lombardy was elaborated for a small area (1. ha) by correlation of borehole lithostratigraphic data down to 160. m below the ground surface. The stratigraphic model was used to compute 1-D synthetic electrostratigraphy based on the petrophysical relationship linking the bulk electrical resistivity of porous sediments to the coarse-to-fine litho-textural ratio and to the average pore-water electrical conductivity. A synthetic apparent resistivity curve was computed for the 1-D synthetic electrostratigraphy and for a traditional Vertical Electrical Sounding with Schlumberger array and a maximum dipole separation of 300. m. A good agreement was observed with the experimental apparent resistivity curve obtained with a Vertical Electrical Sounding collected in the study area. The comparison of the 1-D synthetic electrostratigraphy with the results obtained by inversion of the experimental data with the linear-digital filter method, under the assumption of electrically homogeneous layers and no lateral resistivity transition, was used to estimate the hydrostratigraphic resolving power of ground-based resistivity data at various depths. Stratigraphic units of different hierarchic orders can be resolved by Direct Current methods at different depths and at different sites. In this specific case study, Vertical Electrical Sounding resolution was comparable to the hierarchy of the genetic depositional systems, corresponding to the rank of the hydrostratigraphic systems.